Command-resistant barnyardgrass: testing and treatments

Incidents of herbicide-resistant barnyardgrass in Arkansas are common and destined to become even more so. A new population, the second suspected of being resistant to Command, has been found in a rice field near the Missouri border.

Unable to officially declare the population resistant in mid-May, Jason Norsworthy is still “extremely suspicious” of the biotype.

“I haven’t had the chance to run the dose-response curve,” says the research weed scientist and assistant professor at the University of Arkansas. “We’re in the process of doing that. We’ve looked at individual pots three times in the greenhouse. Each time, Command failed at field-use rates on this biotype.”

Norsworthy is “99 percent sure” the new biotype is just as resistant — if not more — than the earlier find.

“We’re probably two weeks from being able to say the biotype is positively resistant. The plants in the greenhouse were sprayed 14 days ago. We applied lower-than-label rates and higher-than-label rates to see how the plants respond relative to other populations. It’s safe to say the 1X rate is failing and some 2X rate applications are failing as well.”

Late last year, seed from a field near Delaplaine in northeast Arkansas was sent to Norsworthy, who wasn’t told what to screen the weed against.

“When samples come in, we screen them against eight different rice herbicides. I noticed the biotype had trouble with Command and that turned out to be what was being used so much in the field. It all fits.

“After Command failed on it a third time in the greenhouse, I called (the seed supplier) and said, ‘This may be the real deal, here. Can you give me the cropping history?'”

The seed was from an operation close to the supplier. “The best he could tell is the field had been growing continuous rice and, for around the last six years, had been sprayed with Command exclusively.”

The barnyardgrass population in the field wasn’t large. As a result, the farmer would put out Command pre-emergence, “walk away, and call it good. That’s the worst thing from a resistance-management standpoint. Anyway, that grass population gradually grew worse. It still isn’t unbearable but alarming enough for the (seed supplier to notice) and collect some plants.”

After last year’s find, Norsworthy was hoping the Command resistance “was just an isolated case. Now, I’m worried this could be ready to start jumping. It isn’t good to find another resistant population so shortly after the first.”

Testing

The number of weed seed samples being sent in for testing shows “farmers are really concerned about resistance,” says Bob Scott, Arkansas Extension weed specialist. “They’re asking us to run more and more. Right now, the number of samples coming in claiming propanil and/or Facet resistance — or both — is rather staggering.”

The bulk of barnyardgrass samples used to come largely from Poinsett County. Recently “we’re getting calls from Prairie County, Arkansas County and others where previously propanil and Facet were working well.”

The greenhouse screening takes time and patience. “Frankly, many folks believe there’s a magic box we stick the plants in and a ‘plus’ or ‘minus’ lights up on the side indicating resistance or not. If only it was that easy.

“No, we have to grow the plants out in a greenhouse and spray them with a herbicide. And they must be compared to a susceptible plant.”

And in the case of Command resistance, testing has to be done with a soil application. The plants are irrigated to try to mimic a natural system as much as possible.

With late-planted crops and frequent rains, Ford Baldwin — Arkansas weed scientist, consultant and Delta Farm Press contributor — believes this year will be a “throw-away-the-book year for weed control. But the Command-resistant barnyardgrass situation shows you can’t take the potential lightly. It’s like Palmer amaranth (pigweed) in the sense of massive genetic diversity. It can adapt very quickly.”

Treatment

The Command resistance that’s showing up is manageable, says Baldwin. “The overall picture isn’t rosy, though, and a serious reckoning is coming on. To me, resistance in barnyardgrass is the biggest weed threat in rice. It’s even bigger than red rice.”

Asked about control options in barnyardgrass, Ronnie Helms says producers must take into account “we already have propanil-resistant barnyardgrass, Facet-resistant barnyardgrass and cross resistance with propanil/Facet. Now, Command is in the mix.”

Producers “always come up with their own ‘magic mix’ of herbicides to control weeds on the bulk of their acreage,” says Helms, farmer, researcher and consultant with G&H Associates south of Stuttgart, Ark. “They gain a comfort level with certain products. But because of these resistance issues, we can’t do that any longer. We’ll have to integrate other modes of action into the weed management program.”

Helms names Ricestar HT and Grasp as good products for barnyardgrass control. Regiment or Clincher can be used post-flood on escapes. Newpath is also available — “when able, we use a lot of Newpath/Command or Clearpath/Command. (Newpath and Clearpath are for Clearfield rice only.)

“That’s all good because in a rice field there’s usually sprangletop, barnyardgrass, grass weed species, broadleaves, aquatic weeds, and sedges. There’s a diverse population of weeds in rice and two-, three-, or four-way mixes aren’t uncommon. And when you use those mixes, you’re helping reduce the risk of resistance.”

One concoction that Helms employs may be new to some producers: Ricestar HT with Regiment. “By doing that, we’re controlling everything in the field and even have enough activity to bang up yellow nutsedge. Regiment is good on grasses and great on broadleaves. Ricestar HT is good for barnyardgrass and sprangletop and has some activity on broadleaf signalgrass and crabgrass. Put them together and it’s a nice mix using two modes of action that, as far as we know, have no resistance.”

Like Helms, Baldwin likes getting residual herbicides into weed control programs. “Unfortunately, there aren’t that many residuals. With the patent off Facet, the price has dropped considerably and made it more attractive for some growers who don’t have Facet-resistant barnyardgrass. Some are using a Prowl/Facet delayed pre-emergence approach like we used before Command came along.

“That was always an excellent treatment. Folks tended to prefer Command because it was cheaper, a little more consistent and didn’t take as much water management.”

And Command has hardly worn out its welcome and usefulness. “Even though we’ve had a couple of Command-resistant fields show up, we’re telling growers to continue to use Command as a soil-applied program,” says Scott. “For the money, it’s one of the best wide-spectrum grass controls available. We’re relying on products like Ricestar to come back with ACCase inhibitors.”

Another suggestion is using Facet as a soil-applied program instead of post. “Our research has shown that barnyardgrass that Facet won’t kill post-emergence can be controlled much better with Facet applied as a pre-emergence.”

No patterns

The main thing about resistance is to not establish a pattern, said all interviewed.

“I used to regularly say, ‘If it’s not broke, don’t fix it,’” says Baldwin. “Well, I was wrong. That’s a bad policy when it comes to resistance management.

“This barnyardgrass resistance is scary. We must have new technology very soon. To get run out of Clearfield rice because of red rice would be an awful shame. But we’ve been fighting red rice almost since the beginning of rice production. It hasn’t put very many out of business yet. Barnyardgrass, on the other hand, will put you out of business.”

The progression of barnyardgrass resistance is sobering.

“It used to be, when barnyardgrass was at three or four leaves, you could go out with 4 quarts of Stam on a warm day and that would take care of it. Now, once that grass gets up and growing, we can kill about 80 percent. You go out with another product and kill 80 percent of the 20 percent that was left. If you’re not careful, you’ll still have barnyardgrass in the field at the end of the year. The implications of that are frightening.”